The present invention relates to a process for separating various dialkyl multinuclear aromatic compounds from streams containing their isomers. These isomers are of interest for the production of certain di-substituted aromatics which in turn are employed in the synthesis of liquid crystal polymers and specially polyesters.
The p,p'-dialkyl multinuclear aromatic products most suitable for this process, their respective stream, and shape selective catalysts are outlined in the table below:
______________________________________ Shape Selective Product Stream Material ______________________________________ 2,6-diisopropylbiphenyl Mixed DIPN's Mordenite (DIPN) 4,4'-diisopropylbiphenyl Mixed DIBP's ZSM-12, (DIPBP) mordenite 2,6-dimethylnaphthalene Mixed DMN's ZSM-5 (DMN) 4,4-diethylbiphenyl Mixed DEBP's ZSM-12 2-methyl-6-isopropyl Mixed MIPN's Mordenite naphthalene (MIPN) ______________________________________
Those liquid crystal polymers and specially polyesters would likely be commercially attractive if either dihydroxy or dicarboxy forms of the dialkyl multinuclear aromatic compounds were readily available. Unfortunately, they are not. Viable feedstocks which are convertible into either the dihydroxy or dicarboxy monomers based upon known technology are the compounds listed above.
In manufacturing these dialkyl multinuclear aromatics it is clear that some monoalkyl and trialkyl products and a mix of dialkyl isomers will also be produced. In any crude diethyl multinuclear aromatic product stream, separation of these isomers by thermal distillation is difficult because the boiling points of the respective isomers are very close. Similarly, isomer separation by fractional crystallization using melting points is inefficient and suffers from yield problems because of the loss of the desired product in the mother liquor and because of large recycle streams.
It is taught in U.K. Patent Application No. 2,199,590 filed Nov. 27, 1987, that a specific isomer of dimethylnaphthalene can be separated from other isomers when a zeolite Y containing specific metallic ions is used as an adsorbent in combination with a specific desorbent.
Similarly, the separation 4,4'-dialkylbiphenyls in using mordenites is suggested in Japanese Kokai 89/249,729 (assigned to Nippon Steel Chemical Co.).
The references do not suggest combination processes in which the shape selective material used to synthesize the desired p,p'-dialkyl multinuclear aromatic product in a first step is also -used to separate that product from its accompanying isomers.